If he happens to use Quickbooks for his accounting, he can accept credit cards through his software using a laptop with an internet card connection.  He can also use the card reader available from Intuit or key in the numbers manually.  The card reader works well and saves some on the transaction fee.  I would highly recommend it to avoid fees and mistakes.  Handling the transaction this way will allow him to print an invoice and receipts at the same time.  It's fast and very efficient.

If I were him, I would buy the laptop mount available from Ram Mounts that also holds a portable printer and install it in my tow truck.  Power it from the trucks battery power and he would be all set.  I have processed credit cards for several years through Quickbooks using the card reader, and I really like their system.  It is seamlessly tied into the accounting and it eliminates any further inputs of the data.  Bank reconciling is simple too.  Just enter the charges, swipe the card, print the invoice and receipt, and all of the accounting is done.